1. Field of the Invention
This invention relates to a method and arrangement of providing real time process control by means of multiple communication networks. More particularly, this invention relates to such a process control arrangement which achieves extremely rapid response times for the processes being controlled by utilizing multiple, independent communications networks one of which is configured to operate in a very rapid manner relative to any other of the communication networks.
2. Description of the Prior Art
In the field of process control systems, recent developments in industrial automation have evolved from those in which a single supervisory computer controlled the process to those using a distributed system of dedicated microprocessors each responsible for a small aspect of the operation of the total control system. As a result of this reliance on a more distributed approach to process control, there has arisen a commensurate need to develop and refine communication schemes between the plurality of processors to insure that the process would be completed or executed in a timely manner. One example of such a distributed process control configuration is the Westinghouse WDPF.TM. system which has been applied to a wide range of industrial process control operations and is discussed in European Patent Application Nos. EP 0 132 069 and 0 130 802 which are assigned to the same assignee as the present application. In an application of this type of system to an industrial process control configuration, response times of 0.1 to 1.0 seconds could be adequately provided. However, as with other industrial process control configurations, tightly coupled control systems requiring response times of 25 milliseconds or less could not be adequately provided for. This comes about as a result of the fact that a typical data base management network is concerned not so much with raw communications speed but rather, with overheads associated with the data base management operation and with the communications protocols associated with the communications network. Ideally, the control algorithm or master control program in each distributed process control unit, whether microprocessor, minicomputer or otherwise based, would directly access remote data from other units in the same manner that it accessed local data; that is, by accessing data directly from memory without delay or having to consider the actual source or destination of the data.
In a process control system where it is required to provide for such a relatively rapid response time, there generally exists a wide diversity in the types of data that are being processed and, in the usage of that data. This wide diversity consequently results in a broad spectrum of data transfer rates. Typically, a single communication network can address very high data rates for limited amounts of data or larger amounts of data for proportionally reduced data rates.
An example of a process control system that would require such a wide diversity in the timing constraints for the different types of data would be a rolling mill process control system for a steel rolling operation. In such a system, it can be appreciated that as a work piece, which in this example is a steel slab, is transported from one roll stand to another, the process of rolling that steel slab into the exact gauge of steel desired, requires precise coordination between the speed and positioning information of the respective roller assemblies at these roll stands. In addition to the rapid coordination of the speed and positioning information, the overall rolling mill operation deals as well with information for which the timing is not as critical; an example of such information is the historical data which provides the records of how the process has operated from an overall system viewpoint.
Recognizing this need for the rapid handling of certain information, one example of a prior art process control system that could be applicable, utilizes a communications scheme whereby the initialization of the receiving and transmitting stations is accomplished by means of a connecting link rather than a master control unit, such example being found in U.S. Pat. No. 4,177,450 which issued to P. Sarrand on Dec. 4, 1979. In this patent, the described communication arrangement provides that activity on the connecting link is monitored by each station and, based on a predetermined timing scheme, an initiation signal can be transmitted when there has been a lack of activity on the link. Additionally, this patent discloses that the predetermined timing scheme allows the setting of different take over times for the different stations depending on the priority of the various stations. Though this approach does provide for the rapid handling of some data communications between various stations, the effort necessary to establish the timing scheme would appear to be cumbersome in addition to the fact that the flexibility one might desire for the purpose of modifying the overall station arrangement, is greatly reduced. In addition, this approach only provides for one type of communications handling scheme and does not provide for handling different categories of data in different manners so that a more precise handling of data for certain operations can be done.
A further advantage realized as a result of the progress made in using multiple processors in a distributed processing system is the ability to configure the overall system in a variety of manners that allow for the efficient interconnection of process functions that are separated in operating characteristics as well in physical distance. For example, multiple processor systems have been configured in what is known at a ring network in which the number of processor drops or stations are circularly arranged to tie into a ring shaped communications system. It can be appreciated that in a distributed process control system for an industrial application, the actual length of cable over which the ring communication system is configured can be of such a length as to reduce the efficiency of transmitting information thereover. Accordingly, though the use of the multiple processor configuration for an industrial operation may be spread out over a large factory floor area, care must be taken that the actual physical layout does not adversely impact on the efficient communication of information among the various processing units that make up the system.
Another multiple processor distributed processing configuration that has been utilized is commonly referred to as a star configuration which can be characterized by the use of a central processing unit with multiple satellite processing units extending radially outward therefrom along distinct communications links. In this configuration, typically, the processing unit at the core of the star is designated a master processor through which the communication and processing of all information must be coordinated. In addition to suffering from the need to include lengthy communication lines, such a configuration also suffers in that the passing of information through the master processor further tends to slow down the efficient communication of information between the various processor units.
The desired time for communicating information between the multiple processor units that comprise a distributed processor process control system can be considered at least partially to be a function of the efficiency of using the raw data rate of the actual processor devices or CPUs that make up the individual processor units. It can be appreciated that as response times have become faster, more processor operations such as instructions or data transfers can be performed in a shorter time period if the faster response time is better utilized. Therefore, it would be advantageous if a communications arrangement for a multiple processor distributed processing control system could approach the raw data rate of the processing device as nearly as possible thereby increasing the speed at which data can be communicated between the various processing units of the system. By better utilizing the raw data rate of the processor device for communicating data between the various processor units, a further advantage that can be realized is in the area of error detection and correction techniques. As a faster communication arrangement is achieved, obviously, more data can be transmitted in the critical time necessary to accurately perform the process. Accordingly, redundant communications can be sent thereby offering the advantage of providing an inherent error detection and correction technique. Additionally, the ability to send more data in the critical time period also allows the inclusion of data checking information along with the basic data being transmitted.